Here are some great (official) resources on road signs in the United States of America.

Manual on Uniform Traffic Control Devices
From the United States Department of Transportation
Federal Highway administration

http://mutcd.fhwa.dot.gov/We love to drive! Americans relish the freedom of climbing in a vehicle and hitting the road. And as we drive, we rely on a complex series of visual cues to help us make the journey safely. The signs, signals, and pavement markings that guide us are called traffic control devices. These devices are the language that communicates to drivers along the Nation's roadways. They tell us to slow down for the sharp curve on a two-lane rural byway. They make it possible for us to drive 100 km/h (65 mi/h) on the highway separated from other lanes of traffic by only a narrow yellow line. They tell us when and where to stop, and where we should think twice before we park. And they communicate with us in Mobile and Minneapolis, New Orleans and New York, in Seattle and Savannah.

The Manual on Uniform Traffic Control Devices, or MUTCD defines the standards used by road managers nationwide to install and maintain traffic control devices on all streets and highways. The MUTCD is published by the Federal Highway Administration (FHWA) under 23 Code of Federal Regulations (CFR), Part 655, Subpart F.

The MUTCD audience includes the insurance industry, law enforcement agencies, academic institutions, private industry, and construction and engineering concerns.